In a general manner, guided wave resonators utilize the capacity of a piezoelectric material to deform under the application of an electric field. In the presence of an alternating electrical signal, acoustic waves are generated. When these waves are structurally confined in a part of the structure (for example, a layer), one then speaks of guided waves. The most generic example is that of Lamb waves, which correspond to waves propagating in a plate and confined in this plate, given their reflection at the solid/air interfaces. The confinement can also be brought about through the impossibility of the waves propagating in a given medium because of total internal reflections (analogous to the confinement of light in an optical fiber). It can also be afforded through the use of an acoustic mirror. By inverse piezoelectric effect, the stresses associated with the propagation of an elastic wave lead to the generation of an electric current in the electrodes, this being manifested as an electrical resonance.
These resonators consequently take the form of a piezoelectric layer, optionally deposited on a stack of layers called a Bragg mirror, capable of reflecting acoustic waves, whatever their polarizations, and thus of confining them in the piezoelectric medium. A layer structure suspended or deposited on a Bragg mirror makes it possible to insulate the resonator in the vertical direction and to avoid losses by acoustic radiation in the substrate. Electrodes are positioned either on the surface, or on either side of the piezoelectric layer, so as to excite an electric field in the active material as described elsewhere.
The upper electrodes exhibit the appearance of two inter-digitated combs whose period Λ is equal to a half-wavelength and are produced on the surface of a plate of piezoelectric material P. The lower electrodes, if they exist, can either be limited to an electric plane E2, or be likewise in the form of two inter-digitated combs Ei2. These various resonator structures are represented in FIGS. 1a to 1c respectively.
Guided wave resonator structures are represented in FIGS. 1d to 1f using Bragg mirror structures MR produced on the surface of a substrate S, to confine the acoustic waves in the piezoelectric material. In all cases, the periodicity of the electrodes fixes the resonant frequency, since these two quantities are linked by way of the wave propagation speed:
  λ  =      V    f  
where λ is the wavelength, V the wave propagation speed and f the resonant frequency.
To avoid energy leakages on the edges of the resonators, it is common practice to limit the piezoelectric layer in which the waves propagate by a vertical flank Fv as illustrated in FIG. 2a. The exact disposition of this flank depends on the vibration mode utilized, but must also correspond to a vibration antinode so that the operation of the resonator is disturbed as little as possible. It should be noted that this etching can also be extended more deeply by etching the layers of the Bragg mirror, or indeed the substrate.
It is also possible to use a reflecting array composed of short-circuited electrodes Eilc as illustrated in FIG. 2b in the case of a device comprising a membrane produced on the surface of a substrate S of piezoelectric material P, making it possible to create an air gap Gair ensuring a function of confining the acoustic waves in the piezoelectric material and thereby making it possible to dispense with a Bragg mirror structure.
The electrical response of guided wave resonators is manifested as abrupt variations of the electrical impedance as a function of frequency. A key quantity in components of this type is the piezoelectric coupling coefficient. A measurement of this coefficient is accessible through the following formula:
      k    2    =                    f        a            -              f        r                    f      a      
where fr is the resonant frequency (corresponding to a minimum impedance of the resonator) and fa the anti-resonant frequency (corresponding to a maximum impedance of the resonator).
To produce, for example, a bandpass filter, at least two resonators are generally necessary. A bandpass filter is obtained in a known manner by electrical association or by acoustic coupling of the resonators, these associations or couplings being intended to allow the electrical signal to pass through the complete component only over a certain frequency range, and to prevent its passage over the remainder of the spectrum.
Likewise, it is possible to produce a bank of filters. Such a component associates several filters having approximately the same bandwidth, but all shifted with respect to one another, so as to allow the passage for each of them of just a part of a wider spectrum. A well known example may be to provide a bank of filters making it possible to filter a television or radio channel, for each filter. All the filters having to exhibit the same bandwidth, it is necessary to provide resonators at various frequencies but with comparable piezoelectric coupling coefficients.
Thus on the basis of a guided wave resonator utilizing a thickness vibration mode, denoted TE1 (for Thickness Extensional 1), it is easy to produce filters with different frequencies: it suffices to modify the geometric parameters, and notably the period of the excitation electrodes.
An exemplary stack is summarized in table T1 hereinafter, namely the stack of layers Mo, SiO2, SiN, SiOC, SiN, SiOC constituting a Bragg mirror, the piezoelectric layer being made of AIN covered with SiN acting as passivation layer.
TABLE T1LayerThicknessSiN250 nmAlN1850 nm Mo300 nmSiO2210 nmSiN1500 nm SiOC270 nmSiN1500 nm SiOC270 nmSilicon(considered semi-infinite)
For example, filters at 2 GHz, 1.98 GHz and 1.96 GHz may be produced by coupling two resonators like those described hereinafter:
Resonator 1Resonator 2SiO2 thickness210 nmPeriod10μm8.5μmfr1984.5MHz2000MHzfa1999.5MHz2012.4MHzk20.75%0.61%Filter Centered on 2 GHz
Resonator 1Resonator 2SiO2 thickness210 nmPeriod13.7μm10.4μmfr1958.7MHz1980.6MHzfa1979.7MHz1996.5MHzk21.06%0.80%Filter Centered on 1.98 GHz
Resonator 1Resonator 2SiO2 thickness210 nmPeriod26.7μm13.4μmfr1930.2MHz1960.2MHzfa1960.2MHz1980.9MHzk21.53%1.04%Filter Centered on 1.96 GHz
Nonetheless, in this case a bank of three guided wave filters is produced, exhibiting different bandwidths respectively of 18 MHz to 2000 MHz for the filter F10, of 25 MHz to 1980 MHz for the filter F20 and of 40 MHz to 1960 MHz for the filter F30, as illustrated in FIG. 3.